New Marketo Admin

If you will be the acting Admin then I'd say all of them except those for integrations that you do not have. Not trying to be flippant, but when an issue arises, know where to go immediately will save you considerable angst.

Question becomes which do you need to know first, and I think alot of that depends on where your team is in their deployment.

If you are early on, then users and roles is important, you will want to be very familiar with Channels and tags.

Field Management is key!

We are integrated with SalesForce and the Company I work for has been using Marketo for about a year and a half.  I have been a user since January of this year.  

Thanks for the response Clinton.  I will start exploring.  :)

Along with what Clinton said about user roles, etc. I'd just recommend making sure that all of the Marketo organization makes since and is in order so that as you do integrate and expand, you don't need to work backwards as much.

Also, start exploring the fun stuff you can now implement (treasure box releases etc)!